The present invention relates to a process for post-treating pre-expanded polyolefin resin particles. More particularly, the present invention relates to a process for post-treating pre-expanded polyolefin resin particles which eliminates a danger that pre-expanded polyolefin resin particles, which are pre-expanded by using an inflammable blowing agent, spontaneously ignite in the air.
Recently, for heat insulating materials and the like, including energy absorbers such as an automobile bumper core, there are used foams prepared from pre-expanded polyolefin resin particles by means of, for example, expansion molding in a mold.
The pre-expanded polyolefin resin particles are generally produced as follows:
A pressure vessel is charged with the resin particles, water, and the like. As a blowing agent, a Flon gas (chlorofluoro hydrocarbon) such as dichlorodifluoromethane or dichlorotetrafluoroethane is introduced under pressure into the pressure vessel, and the inner pressure of the vessel is raised by heating to impregnate the resin particles with the blowing agent. Then, the resin particles are released into the atmospheric pressure.
However, the blowing agent, the Flon gas is restricted in its use, because it is said that the blowing agent of the Flon gas diffuses in the air, rises to the stratosphere, and destroys the ozone layer. As a result, the rate that injurious radioactive ray reaches the earth increases, and there is a fear that the injurious radioactive ray causes many cases of diseases such as skin cancer.
Therefore, as a blowing agent substitutive for the Flon gas, an aliphatic hydrocarbon gas such as propane, butane, pentane or hexane becomes popular. However, in case of using the aliphatic hydrocarbon gas as the blowing agent, when the produced pre-expanded particles are allowed to stand in the air, the pre-expanded particles are always in danger that the aliphatic hydrocarbon gas gradually released from the inside of the pre-expanded particles catches fire from static electricity or the like on the surface of the pre-expanded particles, and the pre-expanded particles burst into flames.
An object of the present invention is to provide a process capable of giving pre-expanded particles without involving the danger that the blowing agent released from the inside of the pre-expanded particles catches fire from sparks generated on the surface of the particles due to static electricity, and the pre-expanded particles burst into flames.
This and other objects of the present invention will become apparent from the description hereinafter.